The present invention relates to a method and system for damping vibrations in a wind turbine tower.
Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor or through the use of a gearbox. The hub, gearbox (if present), generator and other systems are usually mounted in a nacelle on top of a wind turbine tower.
Various configurations of wind turbine towers are known. Most modern wind turbines comprise a tubular tower structure. Said tubular structure may be made from steel and/or concrete, and may be made from a single segment or may comprise various segments. Other wind turbine towers are made from truss structures. Furthermore, hybrid wind turbine towers are known, which comprise a combination of the previously described tower concepts.
During operation of a wind turbine, undesired vibrations may occur in the wind turbine tower structure. Vibrations in this sense are meant to include any kind of oscillatory or repeating displacements in any direction (transverse, longitudinal or torsional) of any amplitude (large or small) and of any frequency (high or low, constant or varying). These vibrations may be caused by different factors, e.g. wind acting on the tower, blades passing along the tower and locally disturbing the wind flow, vibrations transmitted from the gearbox to the tower, rotor movements, nacelle imbalances, vibrations from the hub transmitted to the tower etc. If a tower is subjected to this kind of vibrations during a prolonged period of time, fatigue damage may result. Fatigue damage may lead to a reduced life time of the wind turbine tower and/or its components. Furthermore, the danger exists that when vibrations cause resonance in the wind turbine tower, this can lead to a potentially dangerous increase of the vibrations.
A further complicating factor is that the size of wind turbines (rotor, nacelle, tower, etc.) keeps increasing. As towers become larger and more slender, they also become more sensitive to induced vibrations.
Also, in the future, wind turbines will be increasingly placed offshore or near-shore. Whether the wind turbine tower is floatingly arranged (offshore) or arranged on a foundation in the sea bed (near-shore), the waves of the sea may form another source of vibrations in the wind turbine tower. Additionally, the design tip speed ratio of wind turbines placed offshore or near-shore is generally higher than for wind turbines placed on shore. The hub will thus rotate at higher speeds. The frequency with which blades pass past the tower thus also increases. The danger of vibrations reaching a resonant frequency of the wind turbine tower increases therewith.
There is thus a clear need to provide a method and system for damping vibrations in wind turbine towers. In the prior art, several systems have been proposed. It is e.g. known to actively control the pitch angle of the rotor blades or the rotational speed of the wind turbine to limit vibrations in the wind turbine tower. However, this means that from time to time the wind turbine is operated in sub-optimal conditions and the electricity generated by the wind turbine is below its potential in order to limit the vibrations in the tower. Additionally, if e.g. pitch control is used to limit the vibrations, the pitch motor(s) are more intensively used, which may reduce the lifetime of the pitch motors.
It may further be proposed to simply increase the thickness of the wind turbine tower. However, this increase of material in the tower would certainly increase the cost of the wind turbine and may even complicate the transport of the tower (segments) to location.
It is also known to provide a tuned mass damper in the form of a mass suspended from the nacelle to counteract the 1st fore-aft and 1st side-to-side tower mode. Suspending a mass at the top of the tower may reduce the available space at the top of the tower (where the available space is already very limited). Additionally, this tuned mass damper can only be used for damping vibrations of a single frequency. In some other prior art embodiments, a mass is suspended a bit lower in the tower to damp the 2nd fore-aft and 2nd side-to-side mode. Also in this case, the available space in the tower for other systems is considerably reduced.
EP 1 677 033 discloses a wind turbine including a vibration load reduction system disposed at either the tower or the nacelle. The vibration load reduction system includes a base, at least two columns extending from the base, and a flowable mass located within the base and the at least two columns. This prior art solution thus adds a mass at the top of the tower, which in itself is not desirable.
WO 2008/000265 discloses a wind turbine tower and control means for establishing oscillation control values in the wind turbine and load altering means for optimizing the tower eigen frequency in response to the values from the control means. Said load altering means may include e.g. steel wires or rods arranged inside the wind turbine tower.
EP 1 811 171 discloses a system for damping a displacement of a wind turbine tower comprising a plurality of shock absorbers and a plurality of beams diagonally arranged within the wind turbine tower. This arrangement inherently has a limited ability to damp vibrations.